Subgroup stratification in SCLC is hindered, however, by the wide heterogeneity and low frequency ( < 10%) of most genes mutated in SCLC beyond the two which define this cancer, TP53 and RB1. The repeated failure of past attempts to find effective targeted therapies for SCLC is likely explained by this mutational heterogeneity and reinforces our need to identify subgroups of SCLC patients that have appropriate biomarkers with clinical relevance. This evidence concerns the gene RB1 and small cell lung carcinoma.